Worcestershire Record No. 22 April 2007 pp. 16-19
"....there is no mistaking a dipper...." - Birds Britannica
|Dippers Pictures © John Robinson|
This article is based on sixteen years of mild intrusion into the lives of the dippers living along part of Dowles Brook in Wyre Forest. It is not a scientific study - no ringing, no mist-netting, no sitting for long periods in a hide - just a periodic walk-through, typically at intervals of several days, sometimes up to a fortnight, between visits, from which I derived a vivid, albeit incomplete, view of their lives. The visits were part of two wider surveys I was undertaking at the time for the British Trust for Ornithology, which required a number of Dowles Brook waterbird species to be monitored, of which the dipper was one. Nonetheless, the opportunity enabled me to get to know more about this fascinating bird, which has long been a favourite of mine.
Dowles Brook runs through the middle of Wyre Forest in a steep-sided valley, cutting its way through the plateau on which the Forest stands. The mixed soils of the plateau are markedly acidic relieved by occasional pockets of limestone, while those of the valley sides and floor tend to be base-rich. Together they support a wide range of vegetation types and a varied plant and animal structure. There are some extensive conifer plantations in the Dowles catchment area, but acidification, so far as I know, does not seem to be a problem and dipper populations, whose invertebrate food supply would adversely affected by it, have remained relatively stable in numbers in the long term. The valley is heavily wooded, mainly deciduous, interrupted here and there by a scattering of old meadows and largely defunct orchards with occasional dwellings and other buildings.
Dowles Brook flows in a general west-east direction, its north-facing slopes are steeper and shady and damp, while those facing south are warmer and drier and contain the more open areas. A disused railway line runs roughly parallel to the stream for most of its length, its embankments encroaching on more than one dipper territory. The river is subject to flash-flooding after rainstorms, while back-flooding from the Severn in winter inundates the lower reaches up to and beyond the Bewdley-Buttonoak road bridge. In winter and after heavy rain it is fast-flowing but at other times and as summer advances it dwindles to a gentle stream. Its bed is generously rock-strewn with shallow gravelly areas and riffles yielding to deeper pools and gullies, providing a rich habitat for dippers to exploit. Its banks are mostly rocky and steep, especially on the southern, ie. north-facing, side. They are clothed with trees, bushes, brambles and other undergrowth, so much so that, when the trees are in full leaf, the river gives the appearance of flowing through a green tunnel. The banks and the watercourse itself are littered with fallen trees, broken branches of all sizes and substantial accumulations of other "woody debris", a term now appearing increasingly in the literature after recent published research has shown that such material, acting as an obstacle to the flow of water and creating pools and other sheltered niches, has generated food resources and habitat conditions important to fish and invertebrate life. These accumulations of debris alter in size and location from year to year as fresh trees collapse into the river and break down, new litter is added and old matter swept away. Undoubtedly these habitats are important to dippers.
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For much of its length Dowles Brook forms the boundary between Worcestershire and Shropshire. My chosen "plot" (fig. l ) was the section running from the Forestry Enterprise bridge 400 metres or so upstream from the inappropriately-named Experimental Pond (now no longer either experimental or a pond) downstream to its junction with the Severn above Bewdley. The terrain is relatively flat - it falls by only 13 m from 40 asl. at the top end to 27m asl. at the bottom, a distance of 4.5 km. My routine during my survey visits was usually the same. I paid nine or ten visits to the area between early March and late June or early July. Each visit began early, as soon after dawn as possible, to catch the birds at their most active time of day. My route took me as close as I could manage to the river (path or no path!), recording on a field map everything I needed for the survey as I went along. Later I collated the information gathered on each visit into an overall season's map and at the end of the season worked out the individual breeding territories.
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Theoretically, dipper territories are relatively easy for an observer to determine. The birds are conspicuous and easily identified, they are strongly territorial and their territories are linear, following the course of the river and any occasional side stream, but not straying into any adjoining country. However, I soon found that things were not quite so straightforward as they seemed. After all, one un-ringed dipper is very like another so problems of identity are frequent. Flush a dipper and it almost invariably flies round the bend and out of sight. Is the next dipper I see the same one or a different one? Am I still in the first dipper's territory or am I in the next one? And so on. Time was always at a premium on my visits and some territories were assumed to exist without the clinching evidence of a proven nest-site. One particular technique I used was to gently flush a dipper along its territory in short flights (this needed some tolerance from the dipper!) and, if I was lucky, the bird would at some point refuse to go on but instead would turn and fly back past me. If this happened once it was of little importance but, if the maneuver was repeated on later visits, I had probably found the territorial boundary I was looking for (fig.2). There were other clues as well and a reasonable conclusion could always be reached once all the field notes for the season were available.
The Dipper Cinclus cinclus, or, strictly speaking, the White-throated Dipper, to distinguish it from the four other dipper species world-wide, is a highly specialised bird. Its generic name Cinctus literally means a bird with a moving tail. The cocked tail of the dipper is a characteristic feature and it is used also in courtship display. The common English name 'dipper' is a reference to its habit of bobbing or 'dipping' often when anxious or alarmed. Two of its many old local names are water ouzel or water thrush as it has a look about it of a small chunky thrush (a family to which it is in fact related). Other European names refer to its habit of 'splashing ' or 'diving' and, as in Britain, to its similarity to a thrush.
Dippers are unique among passerine birds in having evolved to an aquatic lifestyle, to swim and dive under water and to walk along the riverbed and to exploit aquatic invertebrates as prey. It is superbly adapted for this role. Its plumage is dense and heavily waterproofed, its legs are long for its size, its feet large and strong to grip the riverbed firmly and its wings are short and powerfully muscled for negotiating strong currents and as an aid to keeping it on the riverbed when seeking food. Additionally, its eyes are adapted to see equally well above and below the surface of the water (an ability it shares with the cormorant). It also has the odd habit of 'blinking' its eye by rapidly lowering and raising the whitish upper eyelid (originally thought to be the nictitating membrane), the function of which is not entirely clear but could be some form of signal.
Dippers are synonymous with the dramatic mountain scenery and upland rivers and streams of western and northern Britain. However, in certain parts of the country on the eastern fringes of the high ground, where the conditions are suitable, dippers come down to lower altitudes. Luckily, Dowles Brook has over the centuries proved attractive to them. With their aquatic adaptations they seek out their underwater prey, mainly invertebrates and in particular the larval stage of certain insect species, though their diet does include molluscs, crustaceans and some small fish. Larval caddis flies (Trichoptera) and nymphal stoneflies (Plecoptera) and mayflies (Ephemeroptera) figure largely in their prey choice.
Prior to beginning to hunt for food, the dipper usually takes a preliminary look, poking its head half under the surface and then plunging unhesitatingly into the stream however swift the current. They can be seen at their best when working in shallow water, moving busily around, disappearing every now and then and then suddenly reappearing some distance away only to dive back in again as the search continues, looking for all the world like some small mammal rather than a bird. As a non-expert I was not able to keep track of what the dippers were eating although their efforts with caddis fly larvae were easy to spot, the dipper emerging from below with the prey in its casing held firmly in its bill, to be brought ashore and hammered against a stone to dislodge the larva which was promptly consumed. And all done with remarkable skill and dexterity.
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Dippers begin breeding early in the year and will mostly have established their territories well before my arrival in the first few days of March. Their territories, confined to the course of the river, vary in length between breeding pairs (fig.3), the size of the territory determined principally by the availability of food. It is a fact that the most consistently successful pair on Dowles Brook occupied geographically the smallest territory. It must have been rich in invertebrates!
Male dippers are marginally larger than females, though this is not always apparent in the field, even when the pair are together. Both male and female sing though the female has almost given up by early March except during courtship. Then the male displays before the female, dipping, wing-shivering, blinking and giving a distinctive sharp rattling call, quite unlike his usual call-note. In moments of high excitement, both birds will take to flight, zigzagging in and out of the trees on a rollercoaster ride, often high above the stream, the male following closely behind the female and singing. Dippers sing with the bill almost closed unlike most songbirds and the song has been variously described as "somewhat lark-like" or "sweet and thrush-like" or even "a sweet rippling trill recalling a wren". I must confess that I find the dipper's song simply reminds me of a dipper! A jumbled delivery of sweet and harsher notes, surprisingly strong and well capable of penetrating the roar of a stream in full spate. This penetrating quality also applies to the sharp, nasal "zit zit" call-note, sometimes heard when the bird is at rest but more often given in flight, in my experience.
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With territories established, nest-building begins on Dowles in early March when both birds can be seen gathering nesting material. Dippers are very exacting in their choice of nest-sites and, having found a favourable one, show remarkable site fidelity, returning to the same spot, or one very close by, year after year (fig.4). There are on Dowles Brook to the human eye many suitable nesting areas which nonetheless remain unused while the dippers cling stubbornly to their favourite sites. Admittedly, the maverick sites that I have found have not been particularly successful - nests under bridges don't seem to prosper (too much disturbance from foot-passengers?), a location under tree roots was twice used and on each occasion washed out by flooding, one pair built on the near-vertical face of a wall in the open air, the nest sticking out like a sore thumb until it reached a critical size and, when I checked on a subsequent visit, was found to have disappeared. It probably simply fell off the wall. Nests may be in purely natural sites - rocky ledges, crevices in steep banks, tucked in among tree-roots or behind a waterfall or even in overhanging vegetation - or on man-made structures - tunnel walls or piping or against brick, stone or concrete facings or under bridges. All the nests I have found have been sited over running water, seemingly an almost invariable rule with dippers. The north-facing bank on Dowles also seems to be important; all but two of the nests I have ever found have been on this bank. Probably factors already mentioned - steep cliff-sides affording good nest-sites, cool shade - may have been the deciding factor here.
Nest-building can be a long drawn-out affair, especially early in the season. One year I came across a pair building busily in early March but on subsequent visits through to mid-April they seemed to do nothing to the nest, only to start up again six weeks or so after they had begun. The dipper's nest is a large structure, domed and untidy, the bulk of the nesting material being moss which the birds collect in large beakfuls. This is often dipped in the stream before being carried to the nest; possibly the soaking makes the moss more malleable during the building process. Once the mossy outer structure is completed, the inside is lined with grass and roots, finished off finally with a lining of leaves. It is thus possible to gauge the stage the building process has reached by identifying the material being gathered.
Egg-laying usually begins in March, probably near the middle of the month, as young dippers begin to appear fledged and out on the river by late April or early May. I have assumed this timing by the fact that the incubation/fledging period works out at 38-41 days (incubation 16-17 days and fledging 22-24 days) bringing the laying time back to mid-March. The female usually does all the incubation but, once the eggs are hatched, both parents feed the young. Dippers spend very little time at the nest when feeding young, sometimes staying for as little as a couple of seconds. This may be because the young in the nest, especially when well-grown, set up a loud chorus when they sense a parent is in the offing, only falling silent when fed. A brief visit probably keeps the danger of detection by a predator to a minimum. One bird I was watching had developed the odd habit, once it had fed the young, of dropping vertically from the nest directly into the water below before splashing across to the opposite bank and flying off. Once out of the nest, young dippers can swim and dive straight away. They are fed by their parents for ten days or so, after which they are perfectly capable of looking after themselves. They are distinguishable from the adults by dark grey upper parts and dingy creamy underparts, smudged with grey, the patterning of the smudges sometimes varying sufficiently from one bird to another for them to be individually identified.
The underlying purpose of the surveys I was engaged upon was the monitoring of waterbird populations. During my period the dipper population on my plot varied from three to five pairs. To compare this with records for earlier years would need more detailed knowledge of the dippers breeding upstream but from my limited knowledge I would put that population at two, possibly three, pairs at least up as far as Furnace Mill. This would make the estimated total Dowles population as five to eight pairs currently. How does this compare with previous years? An early report suggests inter-war "an estimation of 10 pairs". Post-war, Hicken obviously knew of this figure but thought the level "unlikely" by the early 1970s. There was also an estimate at about this time that there were seven pairs in 1969 "....on the brooks between Bewdley and Holt Fleet...." (of which Dowles would have been the major component). An old map in my possession is not strictly comparable with my plot but it does suggest that the position has not changed greatly since 1987. All this is rather shaky as a basis for a firm opinion but I feel that it does tell us that the dippers have managed to survive reasonably well up to now. A note of caution though. There were only three dipper territories in 2006, my final year, that is at the lower end of the range. This could have been a normal fluctuation from one season to another (it has happened before) but two additional elements have recently emerged which might affect the future. Two territories have been lost. The first was caused by human intrusion when 'renovation' work around an historical nest-site wiped it out completely; the second is a mystery; a site that has enjoyed considerable success in the past despite being near human habitation has been practically deserted for the last two seasons with no sign of nesting activity for no apparent reason. The question is whether the loss of these sites will critically reduce the available supply or whether the dippers can find alternative sites to replace the losses. I'm afraid only time will tell.
Early spring surveys in 2007 undertaken by Steve Davies suggested that four pairs of dippers occupied territories.
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